I rarely carry a separate, dedicated summit pack because my regular pack is already small and light enough to suffice. I just leave whatever I don't need for the summit in camp.

There are circumstances where separate summit packs are entirely safe and appropriate. Every situation is different. Besides, no exact definition of a "summit pack" was offered. IMO the title of this thread as it stands is a sweeping generalization.
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If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.